Researchers at the Department of Energy’s Oak Ridge National Laboratory have developed a method that demonstrates how fiber-reinforced polymer composite materials used in the automotive, aerospace and renewable energy industries can be made stronger and tougher to better withstand mechanical or structural stresses over time.

Composites are strong and lightweight relative to their metallic counterparts. They are also corrosion and fatigue resistant and can be tailored to meet specific industrial performance requirements. However, they are vulnerable to damage from strain because two diverse materials—rigid fibers and a soft matrix, or a binder substance—are combined to make them. The interphase between the two materials needs to be improved because of its influence on the overall mechanical properties of the composites.

The research team deposited thermoplastic nanofibers to create chemically a supportive network that toughens the interphase. Their technique differs from conventional methods of coating the fiber surfaces with polymers or providing a rigid scaffolding to improve bonding between the fiber and the matrix, which have been shown to be inefficient and expensive.

An open-access paper detailing this ORNL-led research is published in Advanced Science.

A schematic illustration of fiber-matrix interphase with PAN nanofibers (≈0.35-µm-diameter) deposited on the ≈5-µm-diameter carbon fiber surface creating a hierarchical architecture covalently bonded with ABS matrix. An in-plane shear strength test was performed to characterize the composites’ interfacial properties. While the representative load-displacement curve of pristine composites (i.e., the composites without any hierarchical nanofibers) exhibit weak behavior (blue curve), the nanofiber-enhanced composites are tougher with higher work of fiber-matrix debonding (red curve). The C─N bond formed between PAN and ABS resulted in a co-continuous interphase between the core fiber and the matrix, resulting in stronger and tougher composites. Gupta et al.

ORNL’s Sumit Gupta, lead author, said he and the team carefully selected the nanofibers and matrix material to create high-surface-area scaffolding or bridging as a load transfer pathway, a mechanism through which stress is passed between the reinforcing fibers and the surrounding matrix material.

Our process enables the material to withstand greater stress. By using this simple, scalable and low-cost approach, we are able to increase the strength of the composites by almost 60% and its toughness by 100%.Sumit Gupta

ORNL’s Christopher Bowland, corresponding author, said future research lies in different fiber and matrix systems that have compatible chemical groups, and the researchers plan to perform more studies on the nanofibers themselves to increase their strength.

p>This study is part of the newly established Composites Core Program 2.0 from the Materials Technology Program at the Vehicle Technologies Office within DOE’s Office of Energy Efficiency and Renewable Energy (VTO-EERE). The program, led by ORNL along with participating labs Pacific Northwest National Laboratory and the National Renewable Energy Laboratory, strives to enhance vehicle efficiency through advanced materials development.

The research team used resources of the Compute and Data Science user facility at ORNL for computational studies to understand the fundamental bonding forces. The team also employed atomic force microscopy at the Center for Nanophase Materials Sciences, or CNMS, to characterize the stiffness or rigidity of the designed interphase. The CNMS is a DOE Office of Science user facility at ORNL.

Resources

• S. Gupta, T. Sohail, M. Checa, S. S. Rohewal, M. D. Toomey, N. Kanbargi, J. T. Damron, L. Collins, L. T. Kearney, A. K. Naskar, C. C. Bowland (2024) “Enhancing Composite Toughness Through Hierarchical Interphase Formation.” Adv. Sci., 11, 2305642. doi: 10.1002/advs.202305642